1. Field of the Invention
This invention relates to processes for the electroplating of chromium from aqueous plating baths and more in particular it relates to the production of chromium electrodeposits from trivalent chromium plating baths. It also relates to novel trivalent chromium plating baths and further to a process for the suppression of chlorine gas and hexavalent chromium formation at the anodes during the operation of such baths.
2. Brief Description of the Prior Art
Chromium has been electrodeposited from trivalent plating baths primarily for decorative purposes. Generally it has not been utilized for functional chromium plating since the adhesion to the substrate has been poorer than the adhesion of chromium electroplates produced from hexavalent chromium plating baths.
The adhesion produced in trivalent chromium plating operations has been sufficient for decorative purposes and provides several advantages as compared to plating from hexavalent chromium plating baths. These include:
1. Higher coverage and freedom from burn at high current densities; PA1 2. Good coverage at low current densities; PA1 3. Reduced sensitivity to current breaks; PA1 4. Fewer problems with waste disposal.
However, in addition to the aforementioned adhesion difficulties, there are several other problems attendant to the use of trivalent chromium plating baths. First, the deposit quality tends to deteriorate rapidly due to the formation of even a few parts per million of hexavalent chromium at the anode. The hexavalent chromium is very detrimental to the quality of the chromium electroplate since black streaks tend to be produced, thereby marring the appearance of the plate.
Further, conventional trivalent chromium plating bath compositions usually include sulfate ions as conductivity anions. Inasmuch as the generally utilized anodes are graphite anodes, oxygen is released during plating operations and can attack the anode causing erosion and the release of carbon particles into the bath. The resultant carbon particles tend to become included in the chromium electrodeposit thereby producing defects in the plate.
It has been suggested that the incorporation of salts containing a halogen anion in the bath will suppress the production of oxygen and thereby yield a better quality electrodeposit with longer bath life. For example, U.S. Pat. No. 3,833,485 suggests that the utilization of relatively high concentrations (0.05 M to 4 M) of either chloride ions or bromide ions in the bath will reduce anode erosion. The minimum useful concentrations are stated to be 0.1 M.
Generally, due to cost factors, chloride ion has been the halogen ion of choice. However, when utilizing effective levels of chloride ions in such plating baths, noxious chlorine gas is given off at the anode.